How to apply stateful computation to a list?

Question

Let's imagine a dummy subset of Brainf*ck:

+ increments the counter

- decrements the counter

A simple program:

program = "++++--" -- should evaluate to 2

And a stateful evaluation function:

eval :: Char -> State Int Char
eval '+' = do x <- get
              put (x + 1)
              return 'I'
eval '-' = do x <- get
              put (x - 1)
              return 'D'

How would you evaluate the program? (Looks like a fold to me but can't get my head around it, and it doesn't feel like it's the way to do it properly...)

Answer 1

You can use traverse_ from Data.Foldable:

import Data.Foldable (traverse_)
execState (traverse_ eval "++++--") 0